[BUG][Java][native] Client - wrong Content-Type, double-quoted payload

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
  https://github.com/cjc343/wrapped-string

• Have you validated the input using an OpenAPI validator (example)?

• What's the version of OpenAPI Generator used?
  4.1.0

• Have you search for related issues/PRs?
  Possibly: #3672

• What's the actual output vs expected output?
  Given a payload of test, the body generated by the client:
  Actual: "test"
  Expected: test

  Given a content definition of text/csv, the Content-Type header:
  Actual: application/json
  Expected: text/csv

Description

• Content-Type header does not match API definition
• Payload is malformed

openapi-generator version

4.1.0, not a regression, first version to support java 11 native client

OpenAPI declaration file content or url

https://github.com/cjc343/wrapped-string/blob/master/src/main/resources/api.yml

Command line used for generation

mvn package

Steps to reproduce

• Run the tests in the sample project

Related issues/PRs

Possibly: #3672

Start with the sample project's API declaration at src/main/resources/api.yml and run its tests with mvn package. Compare the generated Java native client's request body and Content-Type for the test payload and text/csv definition with the expected values. Done means the sample tests show an unquoted payload and the declared Content-Type.